NavList:

   

Reply

Hi Douglass

I use a sextant from about 1920.  A wonderful device, identical to the one 
used by Worsley on his famous boat journey.

Should I be worried about the filtering qualities of my shade glasses?

Would there be a way to check to see if my shade glasses are up to the mark?

Best Regards
Brad

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of douglas.denny@btopenworld.com
Sent: Thursday, June 18, 2009 10:39 AM
To: NavList@fer3.com
Subject: [NavList 8696] Re: Basics of computing sunrise/sunset


NEVER never NEVER look at the sun at any stage of its being above the horizon 
with any from of magnification:  binoculars, telescopes - anything.

I have seen 'holes' in the retina with consequent poor central vision due to this in practice.

Outside of visible spectrum:  especially  infra-red emissions are still there 
and can damage the fovea - permenantly in seconds if there is any intensity 
in the sun's light at all.

Even some so-called "dark glass" or "smoked glass" filters, especially on 
early sextants are suspect for not filtering out the infra-red sufficiently. 
They were not aware of the poor quality of filtering with some 'coloured 
glass filters' outside of visible spectrum in the ninteenth century for 
example, as the physicists like Tyndal and Brewster were only becoming aware 
of the physics.

Make sure you use correct filters whenever using a telescope for viewing the sun.

Douglas Denny.  Optometrist.

Chichester. England.
====================


>
>> "I recall .... using..... binoculars to spot the moment of sunrise/sunset"
>
>>> Readers be warned never to do this if you value your eyesight.
>
> As I see it the sun is an orange (elongated horizontally, actually squashed
> vertically) ball as it is rising or setting, and does not have the magnitude
> it does higher in the sky thanks to the added atmosphere.  Of course lenses
> may magnify its "brightness."  At any rate, what we want to look at is when
> the upper limb is just kissing the horizon (rising or setting) and I do not
> perceive this as a great danger because of atmosphere, refraction, and the
> sliver we can see when the body is physically below the horizon. I am open
> to correction, which is one of the uses of a group like this.
>>


"Confidentiality and Privilege Notice
The information transmitted by this electronic mail (and any attachments) is 
being sent by or on behalf of Tactronics; it is intended for the exclusive 
use of the addressee named above and may constitute information that is 
privileged or confidential or otherwise legally exempt from disclosure. If 
you are not the addressee or an employee or agent responsible for delivering 
this message to same, you are not authorized to retain, read, copy or 
disseminate this electronic mail (or any attachments) or any part thereof. If 
you have received this electronic mail (and any attachments) in error, please 
call us immediately and send written confirmation that same has been deleted 
from your system. Thank you."

--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply